A turbine engine is a device allowing the recovery of energy from a fluid and/or providing energy to a fluid. For example, an expansion turbine may be recovering mechanical energy from a gaseous or liquid flow and thus driving a mechanical system. This energy may also be used to be communicated to another fluid, e.g. for compressing it. A turbine may also be driven by a motor, e.g. an electric motor.
The invention described hereafter has been implemented in the field of turbine engines used for feeding compressed air to a fuel-cell in an auto vehicle. However, other applications may be considered, such as for example a use in an air conditioning system in the aeronautical sector or in the railway sector for example.
In applications such as the ones mentioned above, it is required to have very high rotational rates so as to obtain high efficiency and to avoid a negative influence on the overall efficiency of the fuel-cell (or another system, e.g. an air conditioning system). The rotational rates to consider here are at least 100,000 rpm. At such rotational rates, it is required to have an excellent positioning of the mobile parts with respect to each other and to the able to guarantee that this relative positioning does not evolve with time so as to avoid creating unbalance. Furthermore, it is required to provide, for the rotating parts, contactless bearings, e.g. gas or aerodynamic bearings.
In turbine engines known in the prior art, the mobile assembly consists in an assembly of tubular parts positioned with respect to each other and held in their position by axial clamping ensured by a tie rod crossing at least part of the assembly of parts. US2014/0186745 illustrates, for example, such set up.
Considering the rotational rates of the mobile assembly, balancing this assembly must be performed before operating the turbine engine. When this assembly includes a turbine wheel and a compressor wheel, these wheels generally have a larger diameter than the bearings supporting the assembly. Dismantling must thus be considered for inserting the mobile assembly in its bearings after its balancing. It is required to then keep the precise angular and radial positioning during reassembling so as to avoid unbalance. Tracking is then performed on the parts forming the mobile assembly so as to reassemble them in the same relative position.
WO2013/028521 discloses an air feed device for a fuel cell, having a shaft, a compressor wheel which is arranged in a compressor housing and which is fastened to one of the ends of the shaft, a bearing arrangement which is arranged in a bearing housing for mounting the shaft and an electric motor for driving the shaft, which electric motor is arranged in the bearing housing, wherein the shaft has two shaft bearing portions which are formed as separate components, and has a magnet portion which is arranged between the shaft seat portions and forms a separate component and forms the rotor of the electric motor, the shaft bearing portions and the magnet portion being braced against one another, and the shaft bearing portions and the magnet portion being centered relative to one another by means of a centring arrangement which engages on an outer edge, the shaft bearing portions and the magnet portion, bearing in each case axially against one another.
A disadvantage of an assembly such as presented above is that it includes a number of stacked parts which is relatively high. Therefore, adding the manufacturing and assembling tolerances leads to difficulties to reach the balancing qualities required to operate at high rates. Indeed, this design implies creating an internal momentum which limits reachable rates.
There are also turbine engines having a solid integral shaft with a diameter which is still small so as to reach high rotational rates (see for example U.S. Pat. No. 4,986,733). The shaft ends are threaded and each receives a wheel with the corresponding tapping. Precise wheel positioning is ensured by an intermediate part between the shaft and the wheel. Due to precise machining with tight geometrical and dimensional tolerances, it is thus possible to avoid unbalance when mounting each wheel.
Such turbine engines are not designed to receive a magnet and thus form the rotor of an electric motor. If a magnet is fitted around the solid shaft, reachable rotational rates are below the required rates for air feeding a fuel-cell for example. Furthermore, such structure may only be considered for small shaft end diameters. As soon as the diameter increases, the centrifugal stress and the differential dilatation between a wheel made of light alloy and the shaft made of an alloy with high mechanical characteristics do not allow guaranteeing good relative positioning of the rotational parts.
One objective of the present invention is then to provide a turbine engine architecture integrating an electric motor allowing good balancing with good repeatability of this balancing. Such turbine engine will preferably be compact and/or will have good energy efficiency.